RTC memory values are not maintained after ESP.restart()

gomathisivaraj · March 7, 2025, 7:11am

To maintain some value across softboots, I have been trying to keep some variables in RTC memory. But the values are lost after ESP.restart()
After researching in many places, I found that deepsleep is maintaining the values. Hence I changed to the following. Any idea, Why power is cutoff after restart call? esp_sleep_enable_timer_wakeup(4 * 1000000);
esp_deep_sleep_start();

sivar2311 · March 7, 2025, 7:26am

To place a variable into the RTC memory, you have to define it with the RTC_DATA_ATTR attribute.

RTC_DATA_ATTR int bootCount = 0;

See the example here:

gomathisivaraj · March 7, 2025, 9:18am

I have declared the variable with RTC_DATA_ATTR. I am able to to retain the values across deep-sleep. I expect ESP.restart(), is a soft restart and hence the power will be maintained in RTC memory section and hence I was expecting the values to retain.But it is not.
whereas esp_deep_sleep_start() call is retaining the value.
So my question is about ESP.restart()… and is there any change in ESP-S3, whether restart cuts-off power to RTC are during ESP.restart() call.

sivar2311 · March 7, 2025, 10:24am

I’ve been able to trace ESP.restart() up until it ends into a call to esp_restart_noos() which is declared here:

github.com/espressif/esp-idf

components/esp_system/include/esp_private/system_internal.h

master


      
          /**
           * @brief  Internal function to restart PRO and APP CPUs.
           *
           * @note This function should not be called from FreeRTOS applications.
           *       Use esp_restart instead.
           *
           * This function executes a CPU reset (see TRM).
           *
           * CPU resets do not reset digital peripherals, but this function will
           * manually reset a subset of digital peripherals (depending on target) before
           * carrying out the CPU reset.
           *
           * Memory protection is also cleared by a CPU reset.
           *
           * This is an internal function called by esp_restart. It is called directly
           * by the panic handler and brownout detector interrupt.
           */
          void esp_restart_noos(void) __attribute__((noreturn));

I think the explanation can already be found in the comment:

CPU resets do not reset digital peripherals, but this function will
manually reset a subset of digital peripherals (depending on target) before
carrying out the CPU reset.

It is not mentioned here but i guess that the RTC is one of those peripherals.

Espressif will probably be able to provide you with more detailed answers. A possible starting point would be the ESP-IDF repostiory:

gomathisivaraj · March 16, 2025, 8:52am

Thanks @sivar2311. This line clarifies it.